In an express application request parameters are passed in as first parameter of a route (req, res, next). The sample code posted shows the result of a POST request to a route called /register.
This data does not relate to the model posted with the question.

To be able to work with the model, the data needs to be stored into a new Mongoose object.
So within the route one would write:

Please note that some kind of sanity check is recommended when dealing with user provided variables. Using it like in my example above may enable an attacker to store a string of arbitrary length inside the database which is most probably not desired.

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